The separation of CO2 from typical coal power plant processes can, in principle, be achieved using three different concepts.
Post-Combustion Capture:
In this case, a conventional steam power plant is fed coal and air. Then, a conventional flue gas purification step follows. The CO2 is separated from the flue gas after combustion by using suitable scrubbing steps or, in the longer term, by membrane systems. The disadvantage of this method is that high volume flows of flue gas must be purified with relatively low CO2 concentrations. Membranes for separating the CO2 therefore have high membrane surface requirements. Typical process parameters for the flue gas to be treated would be 1000 m3/s with 18% by volume CO2.
Pre-Combustion Capture:
This method is based on an IGCC (integrated gasification combined cycle) process, wherein the separation of CO2 is carried out in an intermediate step after coal gasification or natural gas reformation, and after the gas purification and gas conditioning (CO shift) steps, but before the combustion step using air. The different coal gasification methods developed so far are preferably operated with oxygen or enriched air (and steam) at a pressure of approximately 20 to 30 bar. For this reason, coal gas has two crucial advantages with respect to CO2 separation. For one, the real volume flow, at a low nitrogen level and high pressure, is approximately 100 times lower than for the flue gases of conventional steam power plants. The direct result is high partial pressures for the main components, CO and H2.
After additional CO conversion into CO2 and H2 by supplying steam (shift reactor) in order to condition the carbon gas for CO2 separation, two options are available, which are the separation of CO2, such as by way of a scrubber, or the separation of a sufficient quantity of H2 using a membrane, wherein gas that is rich in CO2 and suited for liquefaction and storage remains in the retentate. With both options, the hydrogen can subsequently be turned into electric energy in a gas and steam power plant (gas combined cycle), for example, by using an H2 turbine. Typical process parameters after the gas purification would be 10 m3/s with 45% by volume CO2.
Oxyfuel Process:
In this case, simple CO2 separation is carried out by way of condensation after combustion of the coal in a boiler using pure oxygen and a subsequent step of flue gas purification. This method has a crucial advantage. The only combustion products resulting from a combustion process in pure oxygen are CO2 and water vapor, which can be easily separated from CO2 by condensation as the gas mixture cools. The CO2 and water vapor are advantageously recycled in a circuit and recirculated to the boiler together with the oxygen flow. The pure oxygen can either be generated by conventional cryogenic air separation or by using an O2 membrane, wherein the returned (circulated) CO2/water vapor mixture can serve as a flushing gas.
In all three cases, however, no well-functioning concept yet exists for the specific CO2 separation.
High-temperature O2 membranes reportedly have tremendous development potential, particularly in terms of energy. As a condition for this, cost-effective membranes must be available.
For these applications, so-called dense mixed conductors, such as perovskite, may be used. In these, the O2/N2 gas separation is not effected by the separating action of pores, but by the special transport mechanisms in the bulk material. Oxygen ions migrate in the direction of the concentration gradients thereof. On the membrane surface, the electrons leave the oxygen ion and migrate back.
The challenges in the development of the membrane and membrane module as well as in the development of the concept are to achieve the highest degree of separation possible, the highest purity of separated components possible, and the lowest energy expenditure possible during the conditioning of the feed gas and the permeate flow, such as by increasing pressure or using a vacuum. This is intended to achieve low losses in net efficiency, while at the same time achieving the highest flow density possible for the permeating component. At the same time, focus is placed on low surface requirements for the membrane and the lowest apparatus-related costs in the membrane surroundings, thus requiring little additional investment costs. Finally, the desire is to create a module and method concept, which also meets strict requirements in terms of stability and service life, in light of the high operating temperatures.
These requirements are very complex and, in part, contradictory. As a result, high demands in the form of high permeability and selectivity are placed on the membranes used, as well as on the process engineering, in terms of providing favorable process conditions in an optimal membrane separating process, with low additional process engineering costs.
In the problem stated, three fundamental boundary conditions must notably be observed:
1. Non-porous, dense mixed-conductive O2 membranes are subject to a law (Wagner equation), according to which the local O2 permeate flow densities are proportional to the natural logarithm of the particular partial pressure conditions of the permeating component, that is O2 (feed side/permeate side of the membrane). Only in the case of extremely thin membranes are surface effects added to this bulk transport mechanism, whereby the dependency on the pressure conditions is less pronounced.
2. As differs from porous membranes, no special measures are required with respect to the required purity of the O2 product flow, because the O2/N2 selectivity of dense, mixed-conductive membranes is excellent by nature, and is about 100:1 or higher.
3. The inevitably high membrane operating temperature, which is typically 800° C., is a particular challenge with respect to the design and concept. This is further exacerbated for power plant designs that are directed at achieving pressurized operation of the high-temperature membrane and the high-temperature heat exchangers of the membrane surroundings.
As no membrane power plant exists to date, the prior art consists of no more than conceptual proposals in the literature. The concept developments are still in the early stages. The literature discloses basic circuits, however in each case only a single special membrane is examined. Likewise, in graded processes, only one membrane is used for the separations to be carried out on the particular gas flows in the cascade stages. With this individual membrane, the partial pressure of the permeating component decreases continuously, but the feed pressure and permeate pressure are constant over the entire membrane length, unless a flushing gas is used. These pressures can optionally be adjusted by way of a compressor or vacuum pump.
The following concept is known from the prior art for an oxyfuel power plant technology having conventional O2 separation from the air, which is the so-called cryogen air separation plant (LZA) by Vattenfall. Presently, a 30 MWth plant is under construction. FIG. 1 shows a schematic diagram of such an oxyfuel power plant having an upstream air separation system.
In the oxyfuel process, coal is not burned with air, but in an atmosphere of pure oxygen and recycled flue gas. Ash is precipitated in the following treatment steps, as in the conventional power plant process. The fly ash is then separated by dedusting. In the oxyfuel process, a large portion, up to 75% of the flue gas produced during combustion, is recirculated to the boiler in the form of CO2 and water vapor. Sulfur compounds are extracted from the flue gas flow in the form of gypsum as by-products by way of desulfurization. Finally, the remaining water vapor that was added with the coal is condensed out, so that the remaining flue gas comprises almost exclusively pure CO2. The carbon dioxide can then be compressed to more than 100 bar for further use and/or storage.
The disadvantage of this concept is the high energy requirement of the cryogenic air separation system (LZA), whereby a loss of efficiency of at least 10 percentage points (including CO2 liquefaction) is to be expected. A brown coal power plant according to the present state of the art, for example, has a net efficiency of 43%. If, based on this technology, one were to employ the oxyfuel process with the cryogenic air separation system, an efficiency of only 35% would be likely.
A possible variant of the oxyfuel power plant technology with the O2 membrane is presently under development in the OXYCOAL-AC project. A characteristic feature is the membrane mode of operation, using two process engineering measures in order to achieve high propulsive forces for the permeate flow. First, the air on the feed site is compressed to approximately 20 bar in order to increase the O2 partial pressures to approximately 2 to 4 bar, and secondly, flue gas flushing is used in the counter-current on the permeate side (1 bar) in order to lower the O2 partial pressures (approximately 30-300 mbar). This creates the advantage of high local O2 partial pressure conditions of typically 13:1 (4 bar/0.3 bar) or higher.
On the other hand, the pressurized operation and the flue gas membrane flushing result in a number of disadvantages, which can be listed as follows:                two large volume flows must both enter the membrane at the membrane operating temperature, because otherwise the membrane temperature cannot be maintained;        hot flue gas recirculation causes a high recirculated volume flow, because the cooling effect is generally not effective;        hot gas purification is required;        the high pressure gradient between the feed side and permeate side of the membrane results in very high stability requirements for the membrane module;        the high pressure gradient between the feed side and permeate side of the membrane results in very high stability requirements for the high-temperature recuperative heat exchanger used for air preheating;        the residues of the combustion products reach the permeate side of the membrane;        the CO2 atmosphere on the permeate side limits the membrane selection, for example, barium-containing perovskite membranes exhibit the highest O2 flow densities, but have stability problems in a CO2 atmosphere.        